Node-like axonal specializations along demyelinated central nerve fibres: ultrastructural observations.

Ultrastructural examination of long-term (1 month to 1 year) demyelinated axons of the central nervous system (CNS) has revealed the presence of certain features typical of nerve fibres at the node of Ranvier. Focal regions of dense undercoating of the axolemma were observed and these always extended along regions which were intimately associated with the processes of glial cells. In many cases the glial cells were astrocytes, but in some cases the cells resembled Schwann cells. The astrocytic and Schwann cell processes were sometimes finger-like and they thus resembled the normal projections of these cells onto the nodal axolemma of central and peripheral nerve fibres, respectively. Since the undercoated regions which were associated with astrocytic processes were also often remote from any oligodendrocyte or Schwann cell processes, it seems that certain node-like specializations may form or be sustained in the absence of myelin-forming cells.